The international community needs to take urgent action to mitigate unnecessary injuries and mortality of migratory birds due to plastic pollution.
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Together we can help to curb the giant tide of plastic. World Migratory Bird Day is a unique chance to unite efforts in addressing this rapidly growing environmental problem. This year, World Migratory Bird Day is hoping for an even greater success in terms of number of events and actions being taken by dedicated people around the world to spread the word to address the urgent threat of plastic pollution. Local teams in a number of countries around the world, for example, are organizing beach cleaning events to clear plastic from the shorelines as their World Migratory Bird Day contribution.
We are calling for your support in sharing this important environmental message for a better future for birds and people on our interconnected planet. The impact of plastic pollution has been addressed by CMS Resolution The resolution aims at securing a significant reduction of marine pollution and more sustainable management of marine and coastal ecosystems. These experiments have since been amply confirmed by Natterer , of Vienna In , a white stork was found in the German state of Mecklenburg with an arrow made from central African hardwood, which provided some of the earliest evidence of long-distance stork migration.
Migration is the regular seasonal movement, often north and south, undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat, or weather.
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Sometimes, journeys are not termed "true migration" because they are irregular nomadism, invasions, irruptions or in only one direction dispersal, movement of young away from natal area. Migration is marked by its annual seasonality. Approximately of the world's 10, bird species are long-distance migrants. Many bird populations migrate long distances along a flyway. The most common pattern involves flying north in the spring to breed in the temperate or Arctic summer and returning in the autumn to wintering grounds in warmer regions to the south. Of course, in the southern hemisphere the directions are reversed, but there is less land area in the far south to support long-distance migration.
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The primary motivation for migration appears to be food; for example, some hummingbirds choose not to migrate if fed through the winter. This helps diurnal birds to produce larger clutches than related non-migratory species that remain in the tropics.
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As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season. These advantages offset the high stress, physical exertion costs, and other risks of the migration. Predation can be heightened during migration: Eleonora's falcon Falco eleonorae , which breeds on Mediterranean islands, has a very late breeding season, coordinated with the autumn passage of southbound passerine migrants, which it feeds to its young. A similar strategy is adopted by the greater noctule bat , which preys on nocturnal passerine migrants.
Within a species not all populations may be migratory; this is known as "partial migration". The migrating birds bypass the latitudes where other populations may be sedentary, where suitable wintering habitats may already be occupied. This is an example of leap-frog migration. Female chaffinches Fringilla coelebs in Eastern Fennoscandia migrate earlier in the autumn than males do  and the European tits of genera Parus and Cyanistes only migrate their first year. Most migrations begin with the birds starting off in a broad front.
Often, this front narrows into one or more preferred routes termed flyways. These routes typically follow mountain ranges or coastlines, sometimes rivers, and may take advantage of updrafts and other wind patterns or avoid geographical barriers such as large stretches of open water.
The specific routes may be genetically programmed or learned to varying degrees. The routes taken on forward and return migration are often different. Many, if not most, birds migrate in flocks. For larger birds, flying in flocks reduces the energy cost. Birds fly at varying altitudes during migration. An expedition to Mt. Anecdotal reports of them flying much higher have yet to be corroborated with any direct evidence. Bird migration is not limited to birds that can fly. Most species of penguin Spheniscidae migrate by swimming. Dusky grouse Dendragapus obscurus perform altitudinal migration mostly by walking.
Emus Dromaius novaehollandiae in Australia have been observed to undertake long-distance movements on foot during droughts. While participating in nocturnal migration, many birds give 'Nocturnal Flight Calls', which are short, contact-type calls. Nocturnal migrants land in the morning and may feed for a few days before resuming their migration. These birds are referred to as passage migrants in the regions where they occur for a short duration between the origin and destination.
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Nocturnal migrants minimize depredation, avoid overheating, and can feed during the day. Migrants may be able to alter their quality of sleep to compensate for the loss. The typical image of migration is of northern landbirds, such as swallows Hirundinidae and birds of prey, making long flights to the tropics. However, many Holarctic wildfowl and finch Fringillidae species winter in the North Temperate Zone , in regions with milder winters than their summer breeding grounds. For example, the pink-footed goose migrates from Iceland to Britain and neighbouring countries, whilst the dark-eyed junco migrates from subarctic and arctic climates to the contiguous United States  and the American goldfinch from taiga to wintering grounds extending from the American South northwestward to Western Oregon.
The European pied flycatcher Ficedula hypoleuca also follows this migratory trend, breeding in Asia and Europe and wintering in Africa.
Migration routes and wintering grounds are both genetically and traditionally determined depending on the social system of the species. In long-lived, social species such as white storks Ciconia ciconia , flocks are often led by the oldest members and young storks learn the route on their first journey. Often, the migration route of a long-distance migratory bird doesn't follow a straight line between breeding and wintering grounds.
Rather, it could follow a hooked or arched line, with detours around geographical barriers or towards suitable stopover habitat. For most land-birds, such barriers could consist of seas, large water bodies or high mountain ranges, a lack of stopover or feeding sites, or a lack of thermal columns important for broad-winged birds. The same considerations about barriers and detours that apply to long-distance land-bird migration apply to water birds , but in reverse: a large area of land without bodies of water that offer feeding sites may also be a barrier to a bird that feeds in coastal waters.
Detours avoiding such barriers are observed: for example, brent geese Branta bernicla migrating from the Taymyr Peninsula to the Wadden Sea travel via the White Sea coast and the Baltic Sea rather than directly across the Arctic Ocean and northern Scandinavia. A similar situation occurs with waders called shorebirds in North America. Many species, such as dunlin Calidris alpina  and western sandpiper Calidris mauri ,  undertake long movements from their Arctic breeding grounds to warmer locations in the same hemisphere, but others such as semipalmated sandpiper C.
For some species of waders, migration success depends on the availability of certain key food resources at stopover points along the migration route. This gives the migrants an opportunity to refuel for the next leg of the voyage. Some examples of important stopover locations are the Bay of Fundy and Delaware Bay. Seabird migration is similar in pattern to those of the waders and waterfowl. Some, such as the black guillemot Cepphus grylle and some gulls , are quite sedentary; others, such as most terns and auks breeding in the temperate northern hemisphere, move varying distances south in the northern winter.
The Arctic tern Sterna paradisaea has the longest-distance migration of any bird, and sees more daylight than any other, moving from its Arctic breeding grounds to the Antarctic non-breeding areas. Many tubenosed birds breed in the southern hemisphere and migrate north in the southern winter. The most pelagic species, mainly in the 'tubenose' order Procellariiformes , are great wanderers, and the albatrosses of the southern oceans may circle the globe as they ride the "roaring forties" outside the breeding season.
The tubenoses spread widely over large areas of open ocean, but congregate when food becomes available. Some Manx shearwaters Puffinus puffinus do this same journey in reverse. As they are long-lived birds, they may cover enormous distances during their lives; one record-breaking Manx shearwater is calculated to have flown 8 million km 5 million miles during its over year lifespan.
Some large broad-winged birds rely on thermal columns of rising hot air to enable them to soar.
These include many birds of prey such as vultures , eagles , and buzzards , but also storks. These birds migrate in the daytime.
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Migratory species in these groups have great difficulty crossing large bodies of water, since thermals only form over land, and these birds cannot maintain active flight for long distances. Mediterranean and other seas present a major obstacle to soaring birds, which must cross at the narrowest points. Massive numbers of large raptors and storks pass through areas such as the Strait of Messina ,  Gibraltar , Falsterbo , and the Bosphorus at migration times. More common species, such as the European honey buzzard Pernis apivorus , can be counted in hundreds of thousands in autumn.
Other barriers, such as mountain ranges, can also cause funnelling, particularly of large diurnal migrants. This is a notable factor in the Central American migratory bottleneck. Batumi bottleneck in the Caucasus is one of the heaviest migratory funnels on earth. Avoiding flying over the Black Sea surface and across high mountains, hundreds of thousands of soaring birds funnel through an area around the city of Batumi , Georgia. From observing the migration of eleven soaring bird species over the Strait of Gibraltar, species which did not advance their autumn migration dates were those with declining breeding populations in Europe.
Many long-distance migrants appear to be genetically programmed to respond to changing day length. Species that move short distances, however, may not need such a timing mechanism, instead moving in response to local weather conditions. Thus mountain and moorland breeders, such as wallcreeper Tichodroma muraria and white-throated dipper Cinclus cinclus , may move only altitudinally to escape the cold higher ground.
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